Stationary radiographic imaging equipment are employed in medical facilities (e.g., in a radiological department) to capture medical x-ray images on x-ray detectors. Mobile carts can include an x-ray source used to capture (e.g., digital) x-ray images on x-ray detector. Such medical x-ray images can be captured using various techniques such as computed radiography (CR) and digital radiography (DR) in radiographic detectors.
A related art digital radiography (DR) imaging panel acquires image data from a scintillating medium using an array of individual sensors, arranged in a row-by-column matrix, in which each sensor provides a single pixel of image data. Each pixel generally includes a photosensor and a switching element that can be arranged in a co-planar or a vertically integrated manner, as is generally known in the art. In these imaging devices, hydrogenated amorphous silicon (a-Si:H) is commonly used to form the photodiode and the thin-film transistor switch needed for each pixel. In one known imaging arrangement, a frontplane has an array of photosensitive elements, and a backplane has an array of thin-film transistor (TFT) switches.
As a result of the non-single crystalline structure of amorphous silicon, a large density of defect states exists within the photosensor. These defect states trap electrons and holes and release them with a time constant determined mainly by the energy level of the defect state, which is in some cases much longer than an imaging frame time. Generally, only trapped electrons of the photosensors are described herein, but it should be understood that holes can be trapped in a like manner and the same mechanisms apply to holes. Therefore, whenever the electric field within the photosensor/photodiode is perturbed either by electrons generated by light from an x-ray exposure, by the bias voltage being varied, or the like, trapped electrons within the photosensor are redistributed among these defect states, generating a detrapping current with a long time constant at the photosensor terminals.
FIG. 1 is a diagram that shows a perspective view of an area detector according to related art including rows and columns of detector cells in position to receive x-rays passing through a patient during a radiographic procedure. As shown in FIG. 1, an x-ray system 10 that can use an area array 12 can include an x-ray tube 14 collimated to provide an area x-ray beam 16 passing through an area 18 of a patient 20. The beam 16 can be attenuated along its many rays by the internal structure of the patient 20 to then be received by the detector array 12 that can extend generally over a prescribed area (e.g., a plane) perpendicular to the central ray of the x-ray beam 16.
The array 12 can be divided into a plurality of individual cells 22 that can be arranged rectilinearly in columns and rows. As will be understood to those of ordinary skill in the art, the orientation of the columns and rows is arbitrary, however, for clarity of description it will be assumed that the rows extend horizontally and the columns extend vertically.
In exemplary operations, the rows of cells 22 can be scanned one (or more) at a time by scanning circuit 28 so that exposure data from each cell 22 may be read by read-out circuit 30. Each cell 22 can independently measure an intensity of radiation received at its surface and thus the exposure data read-out provides one pixel of information in an image 24 to be displayed on a monitor 26 normally viewed by the user. A bias circuit 32 can control a bias voltage to the cells 22.
Each of the bias circuit 32, the scanning circuit 28, and the read-out circuit 30, can communicate with an acquisition control and image processing circuit 34 that can coordinate operations of the circuits 30, 28 and 32, for example, by use of an electronic processor (not shown). The acquisition control and image processing circuit 34, can also control exemplary examination procedures, and the x-ray tube 14, turning it on and off and controlling the tube current and thus the fluence of x-rays in beam 16 and/or the tube voltage and hence the energy of the x-rays in beam 16.
The acquisition control and image processing circuit 34 can provide image data to the monitor 26, based on the exposure data provided by each cell 22. Alternatively, acquisition control and image processing circuit 34 can manipulate the image data, store raw or processed image data (e.g., at a local or remotely located memory) or export the image data.
Exemplary pixels 22 can include a photo-activated image sensing element and a switching element for reading a signal from the image-sensing element. Image sensing can be performed by direct detection, in which case the image-sensing element directly absorbs the X-rays and converts them into charge carriers. However, in most commercial digital radiography systems, indirect detection is used, in which an intermediate scintillator element converts the X-rays to visible-light photons that can then be sensed by a light-sensitive image-sensing element.
Examples of image sensing elements used in image sensing arrays 12 include various types of photoelectric conversion devices (e.g., photosensors) such as photodiodes (P-N or PIN diodes), photo-capacitors (MIS), photo-transistors or photoconductors. Examples of switching elements used for signal read-out include MOS transistors, bipolar transistors and p-n junction components.
DR detectors with amorphous or poly-crystalline photosensors, such as hydrogenated amorphous-silicon photosensors, require a transition from a zero-power state to a stable state ready for exposure of approximately 1-60 seconds. The time for the transition can be limited by the time required for the trap states in such photosensors to transition from a zero-bias state to a state capable of low-noise and stable operation. After an extended time (e.g., over 5-20 minutes) in the zero-bias state, the traps in such photosensors can reach an equilibrium state corresponding to zero-bias. Upon power-up, such photosensors can transition to a reverse-bias state, where the bias voltage between terminals (e.g., anode and cathode) of a photosensor is typically −3 V>VBIAS>−6V. At VBIAS<−3V the electric field across such a photodiode is insufficient to sweep out photo-generated carriers and at VBIAS>−6V the photodiode dark current can increase rapidly because of field-enhanced thermal carrier generation. Trap occupancy in the reverse bias state is considerably lower than in the zero bias state. In the transition from the zero bias state to the reverse bias state, the traps in such photosensors must emit electrons and holes to the conduction band and valance band, respectively. The emission time constant for electrons and holes to be respectively emitted from trap states to the conduction and valance band depends on the energy difference between the trap energy and the respective band edge. For trap states in the center of the band gap, the emission time constant can be more than 10 seconds. The emission time constant also can be very sensitive to the photosensor temperature. If a capture sequence of one or more exposed images and one or more dark reference images is initiated before the photo sensor traps have reached equilibrium in the powered state, then transient charge from trap emission can be sensed in addition to the photo-generated charge and/or the charge from equilibrium dark current. Calibration of the trap emission charge transient is very complex because of its dependence on such exemplary factors including but not limited to time since last exposure, temperature, and time between the power-up and at least one exposure.
FIG. 2 is a flow chart diagram that shows exemplary power-up operation of a related art wireless, portable detector. Prior to power-up or being turned-on, a portable detector can be in a powered-down state since the end of a previous radiographic exposure series (operation block 210). In clinical applications, the time since the last exposure series can range from less than one minute to several days. Upon receipt of an exposure request (e.g., from the acquisition and control image processing 34) via a command interface (operation block 215), the detector can provide power to the digital logic, upon which time the DR detector loads firmware for the support electronics for the backplane (operation block 220). The detector then enables power for the analog electronics, including the gate drivers 28 and the read-out integrated circuits (ROIC's) 32 (operation block 225). The detector then enters a delay period during which the photosensors are stabilized for image capture (operation block 230). During this delay period the detector can, for example, initiate a global reset during which all gate-lines are powered so as to switch the row-select transistors to a conducting state and a net photodiode bias VBIAS, which is the difference of the bias supplied to the anode and the reference voltage that is supplied to the cathode (e.g., by the read-out circuits 32 such as charge amplifiers).VBIAS=VANODE−VREFERENCE Trap states in a photodiode of the detector would emit electrons and holes to the conduction and valence band, respectively, with the detector approaching equilibrium in a time period of ˜1 sec to 60 sec. As shown in FIG. 2, after a fixed delay time to allow photosensor equilibration, the detector then acknowledges to the acquisition control and image processing 34 an exposure ready signal (operation block 235), after which the generator is permitted to enter an exposure sequence for the detector (operation block 240). After one or more exposed frames, the detector may acquire one or more dark reference frames (operation block 245). Following transmission of the data to the acquisition control and image processing 34, the dark reference frames are subtracted from the exposed frames to yield an image representative of the photo-generated charge. Then, the portable detector can be powered-down since the radiographic exposure series is complete (operation block 255).
Related art detectors such as described above have various short-comings that can reduce quality of the radiographic image and/or interrupt workflows of radiographic image capture (e.g., x-ray technicians). For example, an extended delay period can be required between receipt of an exposure request and readiness to expose. During this time the patient may move, which can require the image to be taken a second time. Further, even delay times as long as several seconds can be insufficient to completely bring the photosensors (e.g., a:Si) to an equilibrium state, since mid-gap states have emission times of tens of seconds. As a result, there can be an offset between subsequent images that can make accurate subtraction of the dark reference frames difficult or impossible. Since such an offset is temperature dependent, calibration can be complex and difficult. In addition, the time between procedures also affects the offset between subsequent frames, particularly if this time is shorter than 60 seconds for the traps in an exemplary photosensor to equilibrate to the zero-bias state, which can further complicate accurate calibration and subtraction of dark reference frames.
Various U.S. patents address problems of large density of defect states of amorphous semiconductor materials (e.g., a-Si) and disclose various methods of operating DR detectors to reduce artifacts produced thereby. See for example, U.S. Pat. No. 5,920,070 (Petrick et al.) or U.S. Pat. No. 7,593,508 (Tsuchiya).
However, there is a need for improvements in the consistency and/or quality of medical x-ray images, particularly when obtained by an x-ray apparatus designed to operate with amorphous or poly-crystalline photosensors DR x-ray detectors.